U.S. Pat. No. 3,979,709 to Daniel P. Healey, Sept. 7, 1976 for ELECTRIC FUSE HAVING A MULTIPLY CASING OF SYNTHETIC-RESIN-GLASS-CLOTH, discloses a new electric fuse, the particular novelty of the fuse consisting in a novel casing material produced by the pultrusion process. The novel casing material has mechanical properties which facilitate performance of certain functions thereof, such as, for instance, closing of a casing, or fuse tube, by a pair of terminal caps or ferrules. Difficulties arose when it came to close the casing, or fuse tube, by terminal plugs which are inserted into the ends of a casing, or fuse tube. In that instance the flexibility of the particular pultruded material described in the above Healey patent allowed the escape of hot products of arcing on blowing of the fuse, which tends to cause electric breakdowns in the surrounding of the fuse.
Fuses as described in the above Healey U.S. Pat. No. 3,979,709 have a great bursting strength but are relatively soft and flexible which results (a) from the presence of one or more intermediate plies of relatively loose non-woven fibers, and (b) from the relatively softness of the synthetic bonding resin, e.g. polyester resins.
It is the prime object of the present invention to provide means allowing to close reliably and inexpensively relatively soft casings, or fuse tubes, e.g. casings as disclosed in the above Healey U.S. Pat. No. 3,979,709, by terminal plugs.
Another object of this invention is to provide means which allow to close casings, or fuse tubes, by terminal plugs but without the pins which have heretofore been used to connect the fuse casings, or fuse tubes, to the terminal plugs. It is well known that this way of affixing terminal plugs to fuse casings is expensive since it involves several sequential operations and a relatively large number of pins and means for insulating and or enlarging the ends of the pins.
According to the aforementioned Healey U.S. Pat. No. 3,979,709 an electric fuse embodying the Healey invention comprises a fusible element, an arc-quenching means surrounding said fusible element, a pair of terminal elements arranged on the end of, and supported by, a casing and conductively interconnected by said fusible element. Said casing includes a first ply formed of non-woven glass fiber mat material having non-uniformly oriented fibers and a second and a third ply arranged on opposite sides of said first ply and sandwiching said first ply. Said second ply and said third ply each are of woven glass fiber fabric and each said second ply and said third ply form an overlap extending in a direction longitudinally of said casing. Said overlap of said second ply and said overlap of said third ply are angularly displaced relative to each other. The aforementioned casing includes a thermosetting resin integrating said first ply, said second ply and said third ply into a laminate. The glass fiber density of said laminate is locally increased in the region of said overlap of said second ply and of said third ply to such an extent that the wall thickness of said casing is virtually uniform along the entire periphery thereof.
If such a casing is merely closed by terminal plugs hot products of arcing are allowed to escape through the narrow gap formed between the inner casing wall and the outer terminal plug wall. A casing of a relatively hard synthetic resin such as melamine resin and convolutely wound fiber glass cloth if of normal thickness does not allow an escape of hot products of arcing. The difference in the behavior of both products may be attributed to the fact that glass cloth melamine tubing is a harder, highly homogenous material, while the above pultruded material is non-homogenous and contains zones where the fiber reinforcement is less than in other zones, and the elasticity of the material is consequently larger in one zone than in another. This allows an expansion of the small gap formed between the terminal plug and the casing, and the consequent escape of products of arcing. To be more specific, it appears that the tubing of Healey is slightly more elastic at the points thereof where the ratio of glass fiber to resin is relatively small.